1. Field of the Invention (Technical Field)
The invention relates to an improved hydrogen-driven hydride heat pump refrigeration and heating apparatus and more particularly to a method and apparatus for improvement of the coefficient of performance of hydride driven heat pump refrigeration and heating systems.
2. Background Art
The use of chloro-fluoro-carbon refrigerants (CFCs) is a significant source of pollution to the global environment, a problem the United States and other countries are endeavoring to minimize through promotion of alternative refrigeration and air conditioning systems. One of the most promising alternatives is the use of hydrogen in the form of hydrides as the refrigerant. While this idea has been studied for a number of years, hydride driven heat pump refrigerators have thus far been unable to perform at levels competitive with conventional freon refrigeration systems.
The hydride heat pump for refrigeration was invented by L. E. Terry (U.S. Pat. No. 4,055,962, November 1977). While development of this system continues, a system which can successfully compete with the performance of conventional refrigeration systems has not, to date, emerged. Consequently, the goal of limiting one of the primary uses of CFCs by the alternate use of hydride refrigeration systems had not been realized.
In hydride heat pump systems currently in use, a low temperature metal hydride (the refrigerant hydride) is coupled to a high temperature hydride (the regenerator hydride) permitting energy to be extracted from the refrigerated space. The energy absorbed at low temperature during the refrigeration step disassociates hydrogen from the refrigerant hydride where it flows into the regenerator hydride, which is at a lower pressure. An important advantage of this system is that these heat pumps operate without moving parts, using hydrogen as the working fluid in a closed cycle which can be repeated indefinitely. This technique is described by D. M. Gruen, M. H. Mendleson, and A. E. Dwight, in "Transition Metal Hydrides," Advances in Chemistry Series #167, American Chemical Society, p. 331 (1978).
In The Journal of the Less Common Metals, Vol. 104, p.307 (1984), M. Nagel, Y. Komazaki, and M. Uchida teach conventional refrigerator configurations based on two metal hydrides, in which energy Q.sub.1 is absorbed by the refrigerant hydride at the refrigerator's low temperature range and the desorbed hydrogen is transferred to the regenerator hydride. See points 10 and 12 in FIG. 1 (prior art) in which the log of the pressure is plotted as a function of the reciprocal of the absolute temperature. The curves in FIG. 1 are van't Hoff curves where the slope represents the enthalpy, .DELTA.H, for absorption and desorption of hydrogen from the hydrides. Following its charging with hydrogen, the regenerator hydride (e.g., LaNi.sub.4.65 Al.sub.0.3) is then heated to 150.degree. C. (point 14 of FIG. 1), where heat Q.sub.3 is supplied from external sources to decompose this hydride and cause hydrogen to charge the refrigerant hydride( e.g., MmNi.sub.4.0 Fe.sub.1 where Mm refers to Misch Metal, a mixture of rare earths) at point 16, where the energy of absorption Q.sub.4 is released. The refrigerant and the regenerator are then cooled to the starting points (point 10 and point 12). The cycle can then be repeated. According to G. G. Libowitz and A. J. Maeland, The Journal of the Less Common Metals, Vol. 131, p. 275, (1987), if the heat losses and the sensible heats are neglected, the maximum theoretical value for the coefficient of performance (COP) of these conventional hydride refrigeration systems is: ##EQU1##
Because the enthalpy of the refrigerant (the numerator) in these systems is always smaller than the enthalpy of the regenerator (the denominator), the COP is always less than 1.